1. Field of the Invention
The present invention relates to a two-wire termination impedance generation circuit of a subscriber circuit.
In a conventional space division (SD) exchange such as a crossbar switchboard, the exchange switches are formed by a large number of metallic contacts. The battery feed to the subscriber circuits mounted in one-to-one correspondence with the subscribers is performed through choke coils (or windings of relays). In the construction generally adopted, the direct current flowing between the line A and the line B forming the speech path (two-wire telephone line) connecting to the subscribers is cut and the line A and the line B are coupled by a capacitor of about 2 .mu.F as a value economically realizable and in consideration of the characteristics.
Therefore, as the two-wire termination impedance of a SD exchange, use has been made of a value of for example 600.OMEGA.+2 .mu.F or 900.OMEGA.+2 .mu.F, but the value of 2 .mu.F has been employed from the viewpoint of practical realizability in the prior art and has no relation at all with the impedance of actual cables.
Current exchanges, however, are changing from the above-mentioned SD exchanges to digital exchanges (electronic exchanges). Accordingly, the exchange switches are also being replaced with a memory (speech path memory) for reading and writing digital speech signals from metallic contacts. Along with this, the internal construction of the subscriber circuits is gradually being replaced with electronic circuits and rapid progress is being made in use of analog LSI's. In the termination method (internal termination) using feedback adopted in LSI technology, a value of 2 .mu.F is not particularly required any more.
Despite this, even current digital exchanges continue to use 600.OMEGA.+2 .mu.F or 900.OMEGA.+2 .mu.F as the two-wire termination impedance for maintaining existing interface conditions. As the conditions for measuring instruments defining AC characteristics, on the other hand, a termination impedance of 600.OMEGA. or 900.OMEGA. not using 2 .mu.F is adopted.
In the final analysis, it is required of subscriber circuits that they be provided with a two-wire termination impedance generation circuit for generating by themselves a two-wire termination impedance corresponding to 600.OMEGA.+2 .mu.F as in the past.
2. Description of the Related Art
As will be explained later in more detail with reference to the drawings, the battery feed for the telephone terminal of a subscriber is performed through a choke coil (CH) and the resistors included in the same and through the telephone line (line A and line B). On the other hand, the subscriber circuit is connected to a speech transmission/reception circuit and sends and receives speed transmission signals T.sub.x and speech reception signals R.sub.x through a transformer (TR). For the transformer (TR), use is made of one which ordinarily stops a direct current. To prevent the direct current, a DC-cut capacitor (2 .mu.F) is connected.
In such a conventional interface, if the secondary side of the transformer (TR) is terminated by 600.OMEGA. or 900.OMEGA., the impedance viewed from the telephone line side becomes 600.OMEGA.+2 .mu.F or 900.OMEGA.+2 .mu.F at the speech band frequency.
However, as mentioned earlier, progress has been made in recent years in the use of electronic circuits for subscriber circuits and the need has arisen for providing circuits in the subscriber circuits for generating the above-mentioned two-wire termination impedance on their own. Various such two-wire termination impedance generation circuits have been proposed, such as in U.S. Pat. No. 4,387,273 (subscriber interface circuit with impedance synthesizer) or Japanese Unexamined Patent Publication (Kokai) No. 60-117989 (subscriber circuit).
Practical two-wire termination impedance generation circuits themselves have been perfected in the above publications etc.
There are many points to be improved in these two-wire termination impedance generation circuits if they are to be truly made practical. In particular, there have been almost no proposals of a two-wire termination impedance generation circuit aimed at the improvement of the frequency characteristic.
The present invention focuses on the following three problems which should be dealt with to improve the frequency characteristics:
(i) First, since a two-wire termination impedance generation circuit is inserted in the subscriber circuit, the frequency characteristics of the speech transmission signal in the conventional subscriber circuit changes from that of a so-called loss circuit at the low band to a gain circuit. PA1 (ii) Second, since a two-wire termination impedance generation circuit is inserted in the subscriber circuit, there is the problem of impairment of the return loss (RL), that is, the AC characteristic showing the quality of the two-wire termination impedance (magnitude of difference from target value). This is due to the fact that the gain versus frequency characteristics of an electronic circuit having the two-wire termination impedance generation circuit as a feedback loop becomes a loss circuit at a high band. PA1 (iii) Third, the amount of feedback through the feedback loop increases much more at high band frequencies and so there is the problem that oscillation etc. are caused and the stability of the circuit is degraded. PA1 (i) the capacitor being comprised of series connected first and second capacitors, PA1 (ii) the resistor being comprised of series connected first and second resistors and with a high band bypass capacitor connected at the intermediate connecting point of the same, and PA1 (iii) the resistor being comprised of a complex termination resistor forming an internal termination impedance and a series connected complex termination resistor and complex termination capacitor connected directly to the telephone line as an external termination impedance.
This problem (iii) is particularly remarkable in an exchange system utilizing a complex termination impedance. In recent exchange systems, the demand has been for an improvement of the overall speech quality in consideration of the characteristics of telephone cables. In place of a conventional two-wire termination impedance comprised of 600.OMEGA.+2 .mu.F or 900.OMEGA.+2 .mu.F, more and more countries are adopting two-wire complex termination impedances. If such two-wire complex termination impedances are generated by the above-mentioned two-wire termination impedance generation circuit, as mentioned above, there is the tendency for a much greater increase in the amount of feedback at high band frequencies.